A transformer is also called a voltage converter or an Xformer, and it serves as a component for transferring electric energy flowing in a primary coil to a secondary coil through electromagnetic induction. The transformer is widely used in not only electric products and electronic products, but also in electric power systems, etc. Such a transformer generally includes a primary coil, a secondary coil, and a core. The primary coil and the secondary coil are each constituted by winding, e.g., a soft copper wire, which has an insulating coating and has a round or rectangular sectional shape, around the core. The core is constituted, for example, by stacking a plurality of thin electrical steel sheets, e.g., silicon steel sheets. The core functions as a magnetic circuit for coupling the primary coil and the secondary coil to each other with mutual inductance. As other related-art transformers, there are, e.g., a transformer including a plurality of secondary coils to be adapted for plural transformation ratios, and a transformer including a tertiary coil for a specific purpose.
One of those transformers is disclosed in, e.g., Patent Literature (PTL) 1. In the transformer disclosed in PTL 1, a strip-like electrical steel sheet is wound and the wound electrical steel sheet is cut in a widthwise direction. After inserting two windings through the cut, cut ends of the wound electrical steel sheet at the cut are abutted and joined to each other, thus closing the cut, while the windings are fixedly held. In the transformer disclosed in PTL 1, the wound electrical steel sheet corresponds to the core, and the windings correspond to the coils.
In the related-art transformers described above, the core is of an annular structure having a circular or square shape, for example, to form a magnetic circuit, which can eliminate a leakage of magnetic flux to the exterior, and which can realize efficient magnetic coupling from the primary coil to the secondary coil. Therefore, when the primary coil and the secondary coil are each fabricated by winding a wire around the core that remains in the annular structure, an operation of winding the wire is complicated because the core has the annular structure, thus causing a limit in increasing productivity. On the other hand, from the viewpoint of facilitating the winding operation, when the winding operation is separately performed on each of plural separated members of the core and the plural members are then joined to each other to form the core of the annular structure, or when the wound electrical steel sheet (core) is cut in the widthwise direction and, after inserting the windings through the cut, the cut ends are jointed to each other to close the cut as described in PTL 1, the joining operation requires to be performed in a manner minimizing the magnetic kiss. In PTL 1, particularly, because the cut ends have to be processed so as to incline at an angle of 50° to 70° with respect to the winding direction, substantial time and labor are needed.